The present invention relates generally to systems and methods for minimizing interference between a magnetic imaging (xe2x80x9cMRIxe2x80x9d) system and other electrical or electronic systems used in conjunction with an MRI system, such as focused ultrasound and/or ultrasound imaging systems, and more particularly to systems and methods for synchronizing changes in operating parameters or other electrical activities, e.g., during focused ultrasound procedures, with the timing of magnetic resonance imaging cycles.
Focused ultrasound systems have been suggested for directing heat to a target tissue region within a patient, such as a cancerous or benign tumor, to necrose or otherwise treat the tissue region with thermal energy. For example, a piezoelectric transducer located outside the patient""s body may be used to focus high intensity acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about twenty kilohertz (20 kHz), and more typically between fifty kiloHertz and five Megahertz (0.05-5 MHz)), at an internal tissue region of a patient to therapeutically treat the tissue region. The ultrasonic waves may be used to ablate a tumor, thereby obviating the need for invasive surgery.
During such procedures, it is often desirable to image the tissues being treated, for example, using magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d). Generally, an MRI system includes a static field magnet, a gradient field amplifier, a radio frequency (xe2x80x9cRFxe2x80x9d) transmitter, and an RF receiver. The magnet includes a region for receiving a patient therein, and provides a static, relatively homogeneous magnetic field over the patient. A gradient field amplifier generates magnetic field gradients that vary the static magnetic field. The RF transmitter transmits RF pulse sequences over the patient to cause the patient""s tissues to emit MR response signals. Raw MR response signals may be sensed by the RF receiver and then passed to a computation unit that computes an MR image, which may then be displayed.
An MRI system may be used to plan a procedure, for example, before a surgical or minimally invasive procedure, such as a focused ultrasound ablation procedure. A patient may initially be scanned in an MRI system to locate a target tissue region and/or to plan a trajectory between an entry point and the tissue region in preparation for a procedure. Once the target tissue region has been identified, MRI may be used during the procedure, for example, to image the tissue region and/or to guide the trajectory of an external ultrasound beam to a target tissue region being treated. In addition, an MRI system may be used to monitor the temperature of the tissue region during the procedure, for example, to ensure that only the target tissue region is destroyed during an ablation procedure without damaging surrounding healthy tissue.
One of the potential problems encountered when using MRI to image a focused ultrasound procedure is interference between the MRI system and the focused ultrasound system. An MRI system may be sensitive to radio frequency (xe2x80x9cRFxe2x80x9d) signals, particularly those within the bandwidth used by the MRI system (which, for 1.5 Tesla MRI systems, generally is centered about sixty three MegaHertz (63 MHz)). In particular, transient signals, such as those used to drive a focused ultrasound system, may generate wide band noise and/or may radiate harmonics within the sensitive range of the MRI system. This noise may interfere with the MRI system, particularly when the RF receiver is activated and detecting MR response signals.
Accordingly, systems and methods for improving the results of magnetic resonance imaging during focused ultrasound procedures would be useful.
The present invention is directed generally to systems that operate in synchronization with a magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d) system, for example, during therapeutic, imaging, diagnostic, and/or other ultrasound procedures. Operation of these systems may be synchronized with operation of the MRI system to minimize interference between the systems. Preferably, systems and methods are provided for performing focused ultrasound procedures being monitored using magnetic resonance imaging, and more particularly to systems and methods for synchronizing active operations, such as changes in sonication parameters, burst transmissions, channel sampling, and the like, during focused ultrasound procedures with the timing of magnetic resonance imaging cycles to minimize interference with sensitive segments of the MRI process.
In accordance with one aspect of the present invention, a system is provided that includes an (xe2x80x9cMRIxe2x80x9d) system, and a focused ultrasound system. The MRI system generally includes a static field magnet for generating a substantially static, homogenous magnetic field, a gradient field amplifier for varying the magnetic field in a predetermined manner, and a radio frequency (xe2x80x9cRFxe2x80x9d) transmitter/receiver. The RF transmitter/receiver may include an RF transmitter for generating RF signals, e.g., pulse sequences, and a separate receiver for detecting MR responses of tissue. Alternatively, the RF transmitter/receiver may be a single device configured to operate alternatively in transmit and receive modes. The MRI system may include an MRI controller for providing a timing sequence or otherwise controlling operation of the RF transmitter/receiver and/or other components of the MRI system.
The focused ultrasound system (xe2x80x9cFUSxe2x80x9d) includes a piezoelectric transducer, drive circuitry coupled to the transducer, and an FUS controller coupled to the drive circuitry. The drive circuitry is configured for providing drive signals to the transducer, which may include one or more transducer elements, such that the transducer emits acoustic energy towards a target tissue region within the patient""s body.
The FUS controller is configured for controlling the drive circuitry to change parameters of the drive signals or otherwise activate the focused ultrasound system at one or more times during the timing sequence that substantially minimize interference with the MRI system detecting MR response signals generated by the patient""s body. For example, the FUS controller may determine the timing sequence of the MRI system and control the drive circuitry based upon the timing sequence of the MRI system. Preferably, the FUS controller controls the drive circuitry to change parameters of the drive signals, e.g., frequency, amplitude, and/or phase, and/or perform other transient operations only when the MRI system transmits RF signals. Thus, the FUS controller may maintain the parameters of the drive signals substantially constant when the MRI system is detecting MR response signals emitted by the patient""s body.
In one embodiment, an interface is provided for sampling timing signals generated by the MRI system, the timing signals being used to instruct the RF transmitter/receiver to transmit RF signals or to detect MR response signals. Preferably, the interface includes a cable for connecting to a timing sequence sampling port of the MRI system to the FUS controller. The FUS controller may change parameters of the drive signals only at one or more times during the timing sequence that minimize interference with the MRI system detecting MR response signals, as described above.
In an alternative embodiment, an antenna or other sensor may be coupled to the controller for detecting the RF signals transmitted by the MRI system. The FUS controller may use data obtained by the antenna to determine when the MRI system is beginning or terminating transmission of RF signals and/or detecting MR response signals, and control the focused ultrasound system accordingly.
In a further alternative, the FUS controller may synchronize clocks driving the MRI system and the transducer to obtain a synchronization constant relating clock speeds of the clocks. A delay between when the MRI system is initially activated and when the MRI system begins transmitting RF signals may be determined, e.g., empirically. The FUS controller may then determine the timing sequence of the MRI system by adjusting a predetermined cycle period of the MRI system by the delay and by the synchronization constant.
In accordance with another aspect, a method is provided for performing a focused ultrasound procedure being monitored using magnetic resonance imaging. Radio frequency (RF) pulse sequences are transmitted over the patient""s body, e.g., using an MRI system, such as that described above. A piezoelectric transducer may be driven with electrical drive signals such that the transducer emits acoustic energy towards a target tissue region within the patient""s body. MR response signals generated by tissue within the patient""s body in response to the RF pulse sequences may be detected, e.g., by a receiver of the MRI system. Any activities generating signals that may interfere with the MRI receiver, e.g., by the focused ultrasound system and/or other electrical or electronic devices, may be synchronized with the MRI system, such as changing parameters of the drive signals to the transducer, sampling channels, and the like. Preferably, these xe2x80x9cnoisyxe2x80x9d activities are performed only during the transmit phase of the MRI cycle, thereby substantially minimizing interference with detecting the MR response signals during the receive phase of the MRI cycle.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.